Hazardous host-finding: Comparative study of transmission abilities and predator evasion in parasites

寄生虫是生态系统的重要组成部分。从一个宿主向另一个宿主的成功传播对寄生虫生命周期的完成(从而对其适合性)起着决定性的作用。由于生活周期和环境条件的不同，寄生虫的传播阶段进化出了大小、形状或行为的巨大差异，甚至在密切相关的类群之间也是如此。由于异源吸虫是水生系统中最常见的真核病原体，其自由生活的传播阶段(尾蚴)的大小和形态存在巨大差异，因此双基因吸虫是研究移动传播阶段生态和进化的理想模式系统。此外，最近的研究强调尾蚴作为可能捕食者的重要能源来源的作用。因此，短命的尾虫必须能够在有限的时间窗口内找到并感染合适的宿主，同时躲避捕食者。这两个因素都影响单个寄生虫物种的成功，因此应该会塑造生态系统的寄生虫群落。然而，在自然条件下这种情况在多大程度上发生仍然知之甚少，关键问题仍然没有得到测试，例如捕食者在单个吸虫物种的传播成功中扮演什么角色，以及这是否可以解释在捕食性物种丰富的环境中不同的吸虫群落结构。此外，气候变化被认为对生态系统及其寄生虫有重大影响，一些病原体可能会从这些发展中受益。然而，我们还不知道寄生虫的传播策略将如何受到环境条件变化的影响。该项目的目的是验证尾蚴传播和捕食形成吸虫群落的假设，以及温度变化将对这些过程产生重大影响的假设。为了测试这一点，将在不同的温度情景下进行标准化的实验室和中游实验，以研究自由游动的尾蚴的特定扩散能力及其主要捕食者在淡水环境中的影响。将这些结果与从新西兰奥塔哥采样充分的淡水中获得的关于宿主、寄生虫和捕食者群落结构的现有数据联系起来，将为决定寄生虫扩散并从而塑造这些生态系统寄生虫群落的结构性力量提供宝贵的洞察。

Parasites are important and integral elements of ecosystems. The successful transmission from one host to another plays a decisive role for the completion of the life cycle (and thus for the fitness) of parasites. Due to the differences in life cycles and environmental conditions, parasite transmission stages have evolved a wide diversity of sizes, shapes or behaviors, even among closely related taxa. With 25,000 species parasitizing a variety of hosts, digenetic trematodes are ideal model systems to study the ecology and evolution of mobile transmission stages because they are the most common eukaryotic pathogens in aquatic systems and there is huge variation in the size and morphology of their free-living transmission stages, known as cercariae. Furthermore, recent studies have stressed the role of the cercariae as important energy resources for possible predators. The short-lived cercariae must therefore be able to find and infect a suitable host within a limited time window whilst evading predators. Both factors influence the success of individual parasite species and should thus shape the parasite communities of ecosystems. However, to what extent this happens under natural conditions is still poorly understood and key issues remain untested, e.g. what role predators have on the transmission success of individual trematode species, and whether or not this may explain different trematode community structures in environments rich in predatory species. Furthermore, climate change is believed to have significant impact on ecosystems and its parasites, and some pathogens might benefit from these developments. However, we do not yet know specifically how parasite transmission strategies will be affected by changing environmental conditions. The project aims at testing the hypotheses that cercarial transmission and predation shape trematode communities, and that temperature changes will have significant effects on these processes. In order test this, standardized laboratory and mesocosm experiments on the specific dispersal capabilities of free-swimming cercariae and the effects of their main predators in freshwater environments will be performed under different temperature scenarios. Connecting these results with available data on host, parasite and predator community structure from well-sampled freshwaters in Otago, New Zealand will provide valuable insight into the structuring forces that determine parasite dispersal and thus shape these ecosystems parasite communities.